A traveling-wave ring-resonator is often used in a laser which must operate in a single longitudinal mode, with very low noise. A laser incorporating such a resonator is also useful for providing pump-radiation for optical parametric frequency conversion (frequency division).
Typically a traveling wave ring-resonator includes what is usually termed by practitioners of the art as an “optical-diode”. Such an optical-diode includes a crystal of a magneto-optic material such as terbium gallium garnet (TGG) or an element of terbium-doped glass. The magneto-optic material is used as a unidirectional polarization rotator, in conjunction with polarization selective elements in the resonator, to provide a loss in one direction of circulation in the resonator that is greater than that in the opposite direction. This forces lasing to occur in the resonator only in the direction of circulation in which the loss is least.
Optical-diodes are most effective in a wavelength range between about 400 nanometers (nm) and 1100 nm. The effectiveness of an optical-diode depends on a so-called “Verdet” constant of the magneto-optic material. This constant defines a degree of polarization-rotation, per unit length of the material, per unit applied magnetic field. The Verdet constant for any given material is wavelength-sensitive, and generally decreases with increasing wavelength. In order to compensate for a lower Verdet constant, the path length in the magneto-optic material must be increased, which increase absorption of radiation in the material. This increases the insertion loss of the optical-diode and reduces the power handling capability of the optical-diode due to heating caused by the absorption.
One method and apparatus for reducing the power load on a TGG crystal used for forcing unidirectional lasing in a ring-laser is described in U.S. Pat. No. 8,000,372, granted to R. Russel Austin and Andrea Caprara, and assigned to the assignee of the present invention and incorporated herein by reference. Here, the laser includes a traveling-wave ring-resonator in which one resonator mirror is a mirror of an interferometer. The interferometer includes a TGG crystal and is arranged such that, when the interferometer is operated in an anti-resonant condition, the mirror has a greater effective reflectivity for clockwise circulation of light in the resonator than for anticlockwise circulation of light in the resonator. There is a difference between the clockwise and anticlockwise effective-reflectivity sufficient that lasing in the resonator is possible only in the clockwise direction. The TGG crystal in the interferometer is subject to only about 10% of the power circulating in the ring-resonator.
However effective the Austin-Caprara interferometer may be, if the circulating power is sufficiently high at a particular wavelength, the TGG crystal may still be overloaded. Further, TGG crystals are expensive and difficult to obtain with consistent quality. There is a need for a method and apparatus for forcing unidirectional lasing in a ring-resonator without the need for a TGG crystal, or any other Faraday rotator.